This invention relates in general to anti-lock brake systems (ABS) and in particular to a wheel speed signal input filter for an ABS control module which includes a filter section for attenuating mid-frequency input signals.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve body and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body.
An ABS further includes an electronic control module which has a microprocessor. The control module is electrically coupled to the pump motor, a plurality of solenoid coils associated with the solenoid valves and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The control module is typically mounted upon the control valve body to form a compact unit which is often referred to as an ABS electro-hydraulic control unit.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor and selectively operate the solenoid valves in the control unit to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
The wheel speed sensors are usually passive variable reluctance magnetic transducers which sense the rotation of an associated exciter ring. The exciter ring is formed from a ferrous metal and has a plurality of teeth formed about the circumference thereof. The exciter ring is typically mounted directly upon a vehicle wheel hub or on the ring gear in a differential assembly.
Passive variable reluctance transducers typically include a permanent magnet which is adjacent to an end of a pole piece formed from a ferrous material. The opposite end of the pole piece is shaped as a tip which extends from the transducer and is spaced apart from the exciter ring teeth by an air gap. A coil surrounds the pole piece and is electrically connected to the ABS control module. Examples of prior art variable reluctance magnetic transducers are shown in U.S. Patent Nos. 5,032,790 and 5,486,758.
During operation, a magnetic field extends from the end of the permanent magnet which is adjacent to the pole piece. The magnetic field passes through the pole piece and across the air gap to the exciter ring. A return path for the magnetic field extends through the exciter ring and the air to the end of the permanent magnet which is opposite from the pole piece. As an exciter ring tooth approaches the pole piece tip, the reluctance of the magnetic field decreases, causing the strength of the magnetic field to increase. Conversely, as the exciter ring tooth moves away from the pole piece tip, the reluctance of the magnetic field increases, causing the strength of the magnetic field to decrease. As the magnetic field strength increases, a first voltage is induced across the coil. Similarly, as the magnetic field strength decreases, a second voltage, having a polarity which is opposite to the polarity of the first voltage, is induced across the coil. The passage of successive exciter ring teeth past the pole piece tip induces an alternating voltage across the coil.
The induced alternating voltage is generally proportional to the rate of change of the magnetic field. More specifically, as the exciter ring speed increases, the magnitude of the alternating voltage also increases. Additionally, the frequency of the alternating voltage is directly proportional to the number of exciter ring teeth which pass the tip of the pole piece per unit of time. As described above, the rotation of the exciter ring is associated with the controlled vehicle wheel. Thus, the frequency of the induced alternating voltage is proportional to the rotational speed of the vehicle wheel associated with the exciter ring.
The voltage induced across the transducer coil is supplied to the ABS control module as a speed signal for the controlled vehicle wheel. As described above, the microprocessor in the control module monitors the frequency of the speed signal and is responsive thereto to determine the speed and acceleration of the associated vehicle wheel. However, the speed signal typically includes high frequency noise components which are induced upon the coil by electromagnetic radiation from other vehicle components. These noise components are incompatible with the proper operation of the control module electronics and could adversely affect the operation of the electronics by introducing unwanted signals which could be misinterpreted as speed signals. Accordingly, an ABS input filter circuit which attenuates the components of noise having frequencies above the highest frequencies of interest for ABS operation is usually included between the variable reluctance transducer and the control module electronics.